Coaxial cables are widely used for transmitting analog signals, such as RF cable television signals, to a number of subscribers. These coaxial cables can generally be classified into two distinct categories, namely, trunk and distribution coaxial cables, hereinafter referred to as distribution cables, and coaxial drop cables. Distribution cables and coaxial drop cables generally form very different portions of a coaxial cable communications system and serve different purposes therein.
In particular, distribution cables typically extend across relatively long distances and, accordingly, must transmit the RF signals with a minimum of resistance and attenuation. Consequently, distribution cables provide signal transmission without significant loss in signal strength or integrity. However, since distribution cables do generally extend across relatively long distances, the distribution cables are typically relatively large cables. For example, the outer conductor of a conventional distribution cable can have an outer diameter of 0.50 inches or larger. In addition, distribution cables are relatively stiff in comparison to conventional coaxial drop cables. For example, the minimum bend radius of conventional distribution cables is typically 4 inches or greater. Due, at least in part, to their size and rigidity, distribution cables are generally durable and can physically withstand the rigors of extending across relatively long distances while providing signal transmission having relatively low loss. In addition, although distribution cables are relatively inflexible, distribution cables are not typically required to twist or turn sharply.
In contrast, coaxial drop cables are typically used as the last link in a cable TV system to bring the cable TV signal from a distribution cable, passing near the subscriber's home, directly into the subscriber's home. Coaxial drop cables are generally relatively flexible such that the coaxial drop cables can be readily installed in a variety of environments, such as installation sites, both underground and aerial, in which the coaxial drop cable must be sharply turned and twisted. As known to those skilled in the art, the flexibility of a cable is a measurement of the force required to bend or deform the cable. Coaxial drop cables generally extend from a cable tap to at least one subscriber or subscriber of a communications system. As known to those skilled in the art, a cable tap is operatively connected to a respective distribution cable so as to divert at least a portion of the communications signals to the coaxial drop cable.
Coaxial drop cables generally include a coaxial cable core consisting of a center conductor, typically formed of copper clad steel, and a dielectric material surrounding the center conductor. Coaxial drop cables also include an outer conductor which surrounds the dielectric material. In order to provide a coaxial drop cable with the necessary flexibility, coaxial drop cables typically have an outer conductor which includes a flexible metallic braid overlying a thin overlapping flexible metallic foil wrap. Due to the overlapping construction of the flexible metallic foil wrap, the outer conductor is discontinuous, thereby reducing the shielding provided by the coaxial drop cable. In addition, metallic braid of a conventional coaxial drop cable is particularly leaky electrically at relatively high frequencies. Accordingly, signals may leak from the outer conductor creating interference at the subscriber's home. Alternatively, electrical noise may enter the cable from other electronics at the subscriber's home.
One measure of the shielding provided by a coaxial cable is the shielding effectiveness of the cable. Shielding effectiveness is typically defined as the attenuation of electrical signals in a generally radial direction through the outer conductor of the cable. For example, the shielding effectiveness of conventional coaxial drop cables at radio frequencies is generally between about to 80 dB and about 115 dB. Due, at least in part, to the ingress of noise into the cable, the coaxial drop cable may not be able to support interactive or two-way video, for example.
In addition, the discontinuous outer conductor can allow moisture or other environmental contaminants to reach the cable core, thereby corroding the center and outer conductors. The shielding provided by conventional coaxial drop cables also typically deteriorates with flexure of the cables. For example, the overlapping foil wrap can crack upon flexure of the cable to further decrease the electrical shielding effectiveness of the cable and to increase the possibility of moisture ingress.
Attempts have been made to improve various components of the coaxial drop cable. For example, U.S. Pat. No. 4,691,081 to Gupta et al. discloses a coaxial drop cable having an improved foil wrap comprising a metal foil layer and a polymer supporting layer fusibly bonded directly to the foil shield layer and serving to structurally reinforce the foil shield layer. U.S. Pat. No. 4,701,575 to Gupta et al. discloses a corrosion-inhibiting powder disposed between the outer conductor comprising a foil shield and braided reinforcing covering and the outer protective plastic jacket.
Coaxial cables, including both distribution cables and coaxial drop cables, typically form a portion of a communications system. In addition to the coaxial cable, conventional communications systems generally include a head end or central office which supplies the communications signals, such as RF cable television signals, to the system. The signals can then be transmitted, such as by an optical fiber trunk cable, from the central office to one or more optical node units, typically located in a neighborhood. The optical node unit can, in turn, convert the optical signals transmitted by the fiber optic trunk cable to corresponding electrical signals. Distribution cable, as described above, generally extends from the optical node unit to transmit the electrical signals across relatively long distances within the neighborhood. A cable tap can be connected to the distribution cable to divert at least a portion of the communications signals to one or more subscribers. A coaxial drop cable of the type described above having an outer conductor consisting of a flexible metallic braid overlying a metallic foil wrap typically extends between the cable tap and each subscriber in order to deliver the communication signals to the subscriber. Accordingly, analog signals, such as RF cable television signals, can be delivered to the subscriber via the communications system.
In addition to the communications system, each subscriber also generally subscribes to an electrical utility company for supplying electrical power to the subscriber. The electrical power provided by the electrical utility company is generally transmitted via an electrical distribution system which is separate and distinct from the aforementioned communications system.
Accordingly, while a subscriber can receive telephone signals via the communications system, a conventional wired telephone also requires electrical power in order to operate which must conventionally be obtained via the electrical distribution system. Thus, both the communications system and the electrical distribution system must be operating effectively in order for the subscriber to utilize their telephone.
In some instances, however, the electrical distribution system fails to provide power to a subscriber, including a subscriber's telephone, even though the communications system continues to provide communications signals to the subscriber. Due to the power failure, however, the subscriber is unable to utilize the communications system, such as to place a telephone call. This situation is annoying and, in some instances, dangerous since subscribers oftentimes depend upon telephonic communications for obtaining assistance in an emergency, such as by placing a 911 telephone call. However, in such instances in which the electrical distribution system is at least partially inoperative, the subscriber will be unable to place such an emergency telephone call even though the communications system is operative and available to transmit communications signals.
In view of recent changes in telecommunications architectures, it may also be desirable that a coaxial drop cable also carry high speed digital data communications and two-way video signals, instead of or in addition to conventional telephone service. In addition, for the reasons described above, it may also be desirable to remotely power termination electronics, such as wired telephones and a network interface device, at the subscriber's home. Unfortunately, the braided outer conductor of a conventional coaxial drop cable has relatively high electrical resistance that may preclude the efficient remote powering of termination electronics. In particular, the steel component of the center conductor has a relatively high electrical resistance that can preclude remote powering.
Further, the steel component of the center conductor is relatively hard and therefore has a tendency to create scratches or nicks on the interior surface of a connector as the connector is installed on end portions of the coaxial drop cable. Accordingly, these scratches or nicks may eventually corrode causing an unacceptably high resistance level at the connector. Attenuation of the signal may increase and the electrical resistance may also increase further precluding the ability to carry remote powering on the conductors. The exposed steel end of the cable may also be subject to corrosion.